Silicon wafers are conventionally prepared from a single crystal or a polycrystalline silicon ingot, which typically has a cylindrical shape. The ingot is sliced in a direction normal to its longitudinal axis to produce as many as several hundred thin, disk-shaped wafers. The slicing operation is typically accomplished by means of one or more reciprocating wire saws, the ingot being contacted with the reciprocating wire while a liquid slurry containing abrasive grains, such as silicon carbide, is supplied to a contact area between the ingot and the wire. Conventional wire saw slurries typically comprise a lubricant acting as a suspending and cooling fluid, such as, for example, a mineral oil or some water-soluble liquid (e.g., polyethylene glycol, or PEG).
As the ingot is sliced, the abrasive particles of the slurry are rubbed by the wire saw against the ingot surface, causing silicon particles from the ingot to be removed, as well as metal (e.g., iron) from the wire itself. A significant amount of silicon particles are lost during cutting. The silicon material that accumulates as an ingot is sliced is conventionally known as the “saw kerf.” As the concentration of silicon, as well as other particulate (e.g., metal particulate), in the slurry increases, the efficiency of the slicing operation decreases. Eventually, the slurry becomes ineffective, or “exhausted,” and then it is typically disposed of or discarded. Traditionally, the exhausted slurry has been disposed of by incineration or treated by a waste water treatment facility. However, burning the slurry generates carbon dioxide and sending the slurry to a waste water treatment facility typically results in the formation of a sludge that must be disposed of in a landfill. Accordingly, both approaches of disposal are unfavorable from an environmental point of view, as well as the costs associated therewith. As a result, some have proposed methods by which the abrasive slurry can be recycled and reused. (See, e.g., U.S. Pat. No. 7,223,344, the entire contents of which are incorporated herein by reference for all relevant and consistent purposes.)
However, in addition to the environmental and economic concerns associates with the saw kerf, or exhausted slurry, the loss of potentially useful silicon material should also be considered. Specifically, although wire saw technology has improved, each pass of the wire through the silicon ingot results in the lost of an amount of silicon equivalent to about a 250 to 280 micron thick slice of the ingot. As technology enables thinner and thinner wafers to be slice from the ingot, more and more passes of the wire through the ingot occurs, resulting in more and more loss of silicon to saw kerf. For example, with existing wire saw technologies, kerf loss can represent from about 25% to about 50% of the silicon ingot material.
While there have been some general suggestions of recovering the silicon material from the saw kerf or exhausted slurry for use in, for example, photovoltaic cells (see, e.g., U.S. Pat. No. 6,780,665, the entire contents of which is incorporated herein by reference for all relevant and consistent purposes), there are several drawbacks. For example, the previously known methods do not provide a means of addressing bulk and surface metal contamination that may be present in silicon obtained from the kerf. This can have a significant impact on the purity of the recovered silicon, and subsequently on the end uses that are available for the recovered silicon. Further, the methods of silicon recovery (e.g., froth flotation recovery) used to-date typically do not recover an adequate amount of silicon particles for re-use.
Accordingly, there remains a need for a method to recover and purify silicon particles created by the cutting of silicon ingots so that the recovered silicon can, for example, be melted and recycled for use in various applications, including solar grade silicon material. Optionally, such a method would additionally allow for the recovery of the silicon carbide used in the slurry process, so that it may also be re-used.